This invention relates generally to communications systems, and more particularly, to satellite-based communications systems employing terrestrial repeaters.
Satellite-based communications systems are being developed to provide ubiquitous radio communication services throughout the world. Some satellite-based communications systems include satellites placed in geosynchronous or geostationary orbital slots at altitudes of over twenty-two thousand miles. Other satellite-based communications systems include non-geosynchronous satellites (Non-GEO), such as Low Earth Orbiting (LEO) satellites that are placed at altitudes of a few hundred miles above the earth, and Middle Earth Orbiting (MEO) satellites which are placed at slightly higher altitudes than LEO satellites. The aforementioned satellite-based communications systems provide unique challenges because of the space environment.
One such challenge includes overcoming the problem of signal strength degradation when transmitting communication signals to and from the communication satellites. In part, this signal strength degradation is due to the great distance between the satellite and the receiving station, for example, an individual subscriber unit. The great distance through which the communication signals travel results in a reduction of signal intensity of the received signal due to path loss. In addition, environmental effects, known as fading, further reduce signal intensity due to reflection, refraction, and/or absorption of the transmitted communication signal. Fading is aggravated in regions cluttered by natural obstructions, such as mountainous regions, and in regions cluttered by man-made obstructions, such as the many tall buildings in urban areas. Operation inside buildings results in a particularly high fade that might prevent operation to a communication satellite.
The fade margin is the depth of fade, generally expressed in dB, that a receiver can tolerate while still maintaining acceptable signal quality. Accordingly, designers are continuously developing system components, such as antennas to allow communications systems to tolerate higher fade margins. Unfortunately, such components tend to be both complicated and costly, and antennas used in communications systems with higher fade margins tend to be larger.
In addition, to support higher fade margins, the subscriber units may operate at higher power levels. The high power operation undesirably reduces the battery life of the subscriber units. To reduce the need for the subscriber units to operate at high power levels, repeaters have been employed to perform signal processing functions on incoming radio communication signals, such as recovering, filtering, amplifying, reshaping, retiming, and retransmitting the signal. Unfortunately, repeater design is made complicated by the challenges related to satellite-based communication systems. For example, the timing factors resulting from propagation delay of the radio communication signals between the satellite and the repeater vastly complicates repeater design to solve temporal interference between up-links and down-links.
Signal quality is also degraded by interference from inadequate isolation between the uplink and downlink frequency bands being used for transmitting and receiving communication signals. The inadequate isolation problem is exacerbated when a single main mission antenna (MMA) that both transmits and receives signals is used on the satellite.
Another challenge facing designers of satellite-based communications systems is increasing and maintaining spectral efficiency. Spectral efficiency is the efficiency of a radio communications system in its use of the radio spectrum. A system that is able to efficiently utilize its radio spectrum is more likely to successfully provide radio communication services to a greater number of subscribers than a less efficient system. Thus, a system having high spectral efficiency leads to increased customer satisfaction and increased profitability of the communication system. High spectral efficiency is especially desirable in population dense regions, such as urban areas where the number of subscribers to the satellite-based communication system may be significantly higher than the number of subscribers in a less population dense region.
One technology for increasing spectral efficiency is time division multiple access (TDMA). TDMA is used to allocate a discrete temporal amount of a given frequency band to each subscriber unit in order to permit many simultaneous conversations. Each radio channel is divided into multiple frames, and each frame is divided into multiple time slots, through TDMA. The subscriber unit is then assigned a particular time slot or group of time slots in a frame for transmission.
When the uplink and downlink frequency bands are close together, a time division duplex (TDD) frame structure can be employed to overcome the problems associated with inadequate signal isolation. TDD is a method that employs TDMA for supporting full duplex communications. TDD supports transmission from the subscriber unit to the satellite through one radio frequency channel and one or more specified time slots. Another radio frequency channel and one or more time slots support transmission from the satellite to the subscriber unit. Unfortunately, this TDD approach results in one frequency being idle while the other frequency is used for transmission, which undesirably decreases spectral efficiency.
Accordingly, there is a significant need for a system and method that mitigate the problems associated with signal fading while maintaining spectral efficiency in a satellite-based communications system.